The present invention relates to an apparatus and method for inter-frequency measurement in a spread spectrum communication system.
Wireless communication systems allow a communication link to be established between two or more users without the need for a wired infrastructure to connect these users. This form of communication has increased in popularity over the last few years. However, the frequency spectrum available for wireless communication is a limited resource potentially limiting the use of wireless communication in the long term. To maximise this resource different technologies and techniques have been developed. Cellular radio systems are one example of these technologies where the current systems use at least one of three protocols; frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA).
CDMA has recently attracted considerable interest where multiple communication links can be established over one broad frequency spectrum. Each communication link is assigned a unique spreading code. This allows multiple users to communicate over the same wideband frequency spectrum.
Therefore, in a CDMA communication system a single wideband radio frequency can be assigned to each of the cellular areas that form the communication system, where a plurality of unique spectrum spreading codes are assigned to each area. As such, the number of users that can be multiplexed together is mainly limited by the amount of interference generated by transmissions rather than the radio frequency channels available.
However, there are occasions when more than one wideband radio channel is required in a CDMA cellular system. For example, different operators may assign different frequency bands or an operator may have a plurality of wideband radio channels of different frequencies to support different traffic volumes for different areas. City centres are likely to have a greater number of users than rural areas. To accommodate this larger number more than one wideband CDMA channel may be assigned, where each CDMA channel has it own set of unique codes. FIG. 1 shows four radio zones a to d where zone a is supported by two channels, zone b by three channels, zone c by two channels and zone d by one channel. The number of channels assigned is dependent on the traffic volume for each area.
Therefore, in a CDMA communication system a radio telephone having established a communication link over a particular channel may require to be reassigned to a new channel if the radio telephone moves from one zone to another.
A further example of a hierarchical cell is shown in FIG. 2 which comprises a macro cell 16 with a base station 17 operating at one frequency incorporating a micro cell 18 with a base station 19 operating at a different frequency to that of base station 17. The micro cell may, for example, be an office CDMA communication network. In this example if radio telephone 20 has established a communication link with base station 17 the radio telephone 20 may need to perform an inter-frequency measurement to determine whether a handover to base station 19 should be performed.
A radiotelephone will typically establish a connection with a base station based on signal strength. However, other criteria may be used, for example the cost of the service provided by the different cells.
Once a communication link has been established in a CDMA system between a radio telephone and a base station there is typically a continuous downlink from the base station to the radio telephone. Therefore, if a radio telephone is to perform an inter-frequency measurement the radio telephone receiver will require to be tuned to another frequency potentially resulting in loss of data. This may be unacceptable to the user. One approach to over come this problem has been to use a dual receiver where one receiver is used to perform an inter-frequency measurement while the other receiver is used for receiving data from the established communication link. This approach is disclosed in Finnish patent application 972299. However, a radio telephone may not have two or more receivers or the radio telephone may only be able to use the second receiver for diversity measurement.
In accordance with one aspect of the present invention there is provided a radio telephone for communicating in a spreadspectrum communication system having a plurality of radio channels of different frequencies; the radio telephone comprising communication means for establishing over one of said radio channels a communication link over which is communicated data and associated error correction data; a receiver operable to alternatively receive for a period a radio channel other than the established channel and processing means for measuring the radio signal characteristics of the other channel; the processing means being operable to use error correction data received over the established communication link to calculate the contents of the established communication link data of the radio channel signal during the period.
This invention allows the radio telephone to perform inter-frequency measurements without extra inter-frequency signalling between the base station and the radiotelephone.
This invention allows the radio telephone to perform inter-frequency measurements without the base station needing to know when the inter-frequency measurement is being performed.
Preferably the processing means measures the received signal strength.
Most preferably the communication means is responsive to a control signal from the processing means for requesting the communication system to increase power a predetermined interval before the receiver receives the other radio channel.
Preferably if the signal characteristics of the other radio channel complies with at least one predetermined criterion a communication link over the other radio channel is established.
In accordance with a second aspect of the present invention there is provided a method of measuring a radio channel in a spreadspectrum communication system having a plurality of radio channels of different frequencies; the method comprising establishing a communication link over one of said radio channels over which is communicated data and associated error correction data alternatively receiving for a period a radio channel other than the established channel and measuring the radio characteristics of the other channel using error correction data received over the established communication link to calculate the contents of the established communication link data of the radio channel signal during the period.
Preferably the received signal strength is measured.
Most preferably the communication system is requested to increase power a predetermined interval before receiving the other radio channel.